1. Field of the Invention
The present invention relates to a suspension control system for controlling the attitude of a vehicle such as an automobile.
2. Prior Art
Recently, there has been proposed with an attempt at a practical application, a so-called active suspension system in which control is carried out to counter changes in the attitude of a vehicle which would otherwise occur during cornering, accelerating, braking and so on. A control operation, which is carried out on the basis of signals from vehicle height sensors, a speed sensor, a steering angle sensor, an acceleration sensor and the like, regulates an amount of hydraulic fluid to be supplied to or discharged from suspension units, each disposed between an axle and the vehicle body, to suppress changes in the attitude of the vehicle and thereby stabilize attitude of the vehicle. One example of such an active suspension system is disclosed in Japanese Patent Application Laid Open No. 16409/89.
FIG. 1 shows one example of a prior art active suspension control system. A suspension control system 1 comprises hydraulic cylinders 3a, 3b, 3c and 3d (reference numeral "3" will be used as a general reference numeral thereof hereinafter) mounted on a vehicle body (not shown). Respective piston rods 4a, 4b, 4c and 4d (general reference numeral "4") of the hydraulic cylinders 3 are connected to axles of wheels 2a, 2b, 2c and 2d (general reference numeral "2"), respectively. Accumulators 5a, 5b, 5c and 5d (general reference numeral "5") are connected to the respective cylinders 3. Each pairing of cylinders 3 and accumulators 5 constitutes a suspension unit. The hydraulic cylinders 3 are connected to a pressurized hydraulic fluid source 6 through hydraulic fluid supplying and discharging means 7a, 7b, 7c and 7d (general reference numeral "7"), respectively. Each hydraulic fluid supplying and discharging means 7 comprises an electromagnetic switching valve or a proportional valve which is adapted to be controlled by means of a controller 11 which will be described hereinafter. When electromagnetic switching valves are used as hydraulic fluid supplying and discharging means 7, desired amounts of fluid are supplied to and discharged from the respective cylinders 3 by controlling the periods of time during which the valves are opened. When proportional valves are used, desired amounts of fluid are supplied to and discharged from the respective cylinders 3 by controlling the openings of the valves.
Shown at 10a, 10b, 10c and 10d (general reference numeral "10") are height sensors for measuring the height of the vehicle based on the state of the cylinders 3. Reference numerals 12 and 13 respectively designate a steering angle sensor for detecting the steering angle based on the degree of rotation of the steering wheel and a vehicle speed sensor for detecting the speed of the vehicle. Signals from the sensors 10, 12 and 13 are input to the controller 11. The controller predicts changes in the attitude of the vehicle which would otherwise occur, and controls the hydraulic fluid supplying and discharging means 7 to control the amounts of fluid to be supplied to and discharged from the respective cylinders.
The accumulators 5 are of a gas-sealed-in type, as shown in FIG. 2. In the figure, reference numeral 14 designates a tubular housing with a closed bottom end. Opposite open end of the housing 14 is closed by means of a closure member 15. A piston 18 is slidably disposed in the housing 14 to divide the interior of the housing 14 into a gas chamber 16 and a oil chamber 17. A gas, for example, nitrogen gas is sealed in the gas chamber 16, while the oil chamber 17 communicates with the cylinder 3 through a passage 15a so that the same hydraulic fluid is also introduced into and discharged from the oil chamber 17 when hydraulic fluid is supplied to and discharged from the corresponding cylinder 3 by means of the hydraulic fluid supplying and discharging means. The piston 18 is provided with a sealing member 19 mounted thereon for creating a seal between the gas chamber 16 and oil chamber 17. The piston 18 moves in the housing 14 in response to the supply and discharge control of the hydraulic fluid into and from the suspension unit to provide the suspension unit with spring characteristics.
However, the conventional suspension control system as mentioned above suffers from the following problems.
When a vehicle is running on a rough road, when accelerating while cornering, or when a steering operation is continuously carried out to increase a cornering angle, the height of the vehicle greatly changes. As a result, a great amount of hydraulic fluid is discharged from suspension units and the piston 18 is displaced, thereby contacting the closure member 15. This is called a zero-down state. The zero-down state is accompanied by a harsh noise. Further, the pressure level of the hydraulic fluid in the suspension unit changes suddenly, thereby creating an oil hammer effect, which may result in a deterioration in the durability of hydraulic apparatus such as the cylinders 3, valves of the supplying and discharging means 7 and so on, and thus cause a decrease in driving stability.
In some cases, an annular buffer 20 is mounted on the inner surface of the closure member 15, as shown in FIG. 2, in order to suppress noise. However, it is difficult to suppress the noise over a long period of time due to aging of the buffer.
Further, even if the buffer 20 serves to suppress noise, the arrangement does not serve to eliminate substantial and sudden changes in the pressure level of the hydraulic fluid in the suspension unit. Accordingly, problems of the deterioration of the hydraulic apparatus and of the lowering of the driving stability remain unsolved.